1. Field of the Invention
The present invention relates generally to an apparatus and method for measuring noise power in a high-speed data transmission system, and in particular, to an apparatus and method for measuring thermal noise power in an Evolution-Data Only (1xEV-DO) base transceiver system (BTS).
2. Description of the Related Art
In general, mobile communication systems provide voice service as a basic service and data service as an additional service. Current users of mobile communication systems demand transmission of more data at higher rates from the mobile communication systems. To meet the users' demand, 1xEV-DO has emerged to support high-speed packet data transmission. Like general mobile communication systems, the 1xEV-DO system transmits/receives data to/from mobile stations (MSs) via radio channels. Therefore, a BTS assigns radio resources to the MSs in order to provide service to them. Communications are available through assignment of communication resources, i.e., radio channel resources. This is the most conspicuous difference between mobile communication and wired communication. For wired communication systems, a dedicated line is connected to each terminal irrespective of whether the terminal uses the line. In contrast, in wireless communication systems, communication service is possible only when channel resources are assigned to the mobile terminals in a radio environment. Consequently, if all radio resources are occupied, a new call cannot be connected, and the data rate of a presently on-going call cannot be increased either.
If the BTS assigns more radio resources to some users than available, interference with other users within the BTS increases, which seriously affects the quality of service provided to them. Moreover, the service quality for users serviced in neighboring BTSs also deteriorates. Hence, the BTS must precisely determine currently available radio resources according to channel conditions, and assign radio resources correspondingly. This is one of the very significant roles of the access nodes (AN).
2nd generation (2G) mobile communication systems, which support voice service and low-speed data service, provide traffic data at relatively low rate. A high-speed data transmitting mobile communication system such as 1xEV-DO, however, transmits a large volume of data at high rate on both forward and reverse links. “Forward” is the direction from a BTS to an MS and “reverse” is the direction from the MS to the BS. In the high-speed data transmission, adverse effects on MSs in service in a current BTS and other BTSs are mitigated by reliable assignment and limiting of radio resources. Appropriate assignment of radio resources prevents the degradation of service quality for other MSs, or impossible provisioning of service itself. Numerous studies are being actively conducted on the addition of high-speed data services with the traditional voice calling on reverse links in the present 1xEV-DO system. Service quality is a significant factor to the voice call. In this context, appropriate assignment of radio resources is increasingly important.
Mobile communication systems deployed thus far generally assign low rate radio resources to voice services and slightly higher rates to data services. Thus, radio resources are assigned and limited based on measured reverse link load. Considering the requirement of radio resources for high-speed data transmission, the simple load measuring method is not effective in appropriately limiting the load. To overcome this problem, a method of measuring transmit power on the reverse link separately for a non-silence period and a silence period was proposed for the 1xEV-DO system. The non-silence and silence periods in the 1xEV-DO system will be described in greater below.
FIG. 1 illustrates the non-silence period and the silence period as defined in the 1xEV-DO system. With reference to FIG. 1, the silence and non-silence periods will be described in terms of noise power and available load.
As illustrated, the 1xEV-DO system operates separately in the non-silence period and the silence period. They can be set to alternate periodically according to a time interval set by the system. Alternatively, the silence period can be set to a preset time period. The silence period lasts for a predetermined short time (T). For the silence period, no MSs transmit signals on the reverse link. This implies that there is no power loaded on the reverse link. Even in this state, thermal noise power exists due to radio noise inherent to the environment according to the position of a BS. The power of the silence period illustrated in FIG. 1 corresponds to thermal noise power. An effective load power, as well the thermal noise power, exists for the non-silence period because of power transmitted from the MSs. The effective load power is the load imposed on the reverse link by data transmission from the MSs. This can also be referred to as the rise over thermal (ROT) power illustrated in FIG. 1.
With reference to FIG. 2, power control on the reverse link in a 1xEV-DO BTS will be described. FIG. 2 is a flowchart illustrating a method of controlling the reverse link power in a 1xEV-DO BS.
Referring to FIG. 2, the BTS measures received power on the reverse link in step 200. The power measurement is the total load of the reverse link which includes the ROT and the thermal noise power, as illustrated in FIG. 1. In the former case, an available load is not accurately determined because infrequently-varying thermal noise power is considered a noise variable with control in the BS. Thus, it is preferable to control the reverse load by measuring the ROT. In this sense, the received power measured in step 200 is the ROT.
While measuring the ROT, the BTS decides whether it is time to broadcast an reverse activity bit (RAB) in decision step 202. If it is (“Yes” path from decision step 202), the BTS decides whether the ROT measurement is less than a predetermined threshold in order to control the rates of reverse data from MSs in decision step 204. If the ROT is less than the threshold (“Yes” path from decision step 204), the BTS sets the RAB to “0” to increase the data rates of the MSs in step 206. On the contrary, if the ROT is greater than the threshold (“No” path from decision step 204), the BTS sets the RAB to “1” to decrease the data rates of the MSs in step 208.
After setting the RAB in step 206 or 208, the BTS broadcasts the RAB to all the MSs that transmit data on the reverse link, to thereby control their data rates in step 210.
As described above, the BTS detects the ROT by measuring the received power on the reverse link. The ROT is calculated by subtracting the thermal noise power of the silence period illustrated in FIG. 1 from the total received power measurement. An unchangeable power measuring block (UPMB) used to measure the received power in the BTS measures the instantaneous power of a signal received at an antenna. This UPMB will be described with reference to FIG. 3. FIG. 3 is a block diagram of a typical UPMB for the BS.
Referring to FIG. 3, a UPMB 300 measures the power of a signal received from an antenna (ANT) after predetermined processing or without any processing. In the illustrated case and the following description, it is assumed that there is no processing preceding the power measuring. A power detector 301 of the UPMB 300 measures the received power of the reverse link and outputs the received power at any particular moment. Since peak power is produced instantaneously in a radio environment, power varies greatly from moment to moment. Therefore, the instantaneous power value from the power detector cannot be used without further processing. The UPMB 300 is thus equipped with an average calculator 302 for calculating an average power. The power detector 301 feeds the instantaneous power value to the average calculator 302 and the average calculator 302 accumulates received power values for a predetermined time and calculates the average of the accumulated power values. The average power output of the UPMB 300 is used as a received power measurement.
A controller 311 controls reverse link power and reverse data rates based on the power measurement received from the average calculator 302.
The average calculator 302 can be configured in either a digital or analog circuit. If the average calculator 302 is configured as an analog circuit, as is generally the case, the average calculator 302 is comprised of an integrator with resistors and condensers. Even if the average calculator 302 is configured as a digital circuit, it can calculate the average power when implemented as an integrator. The use of an integrator as the average calculator 302, however, is not capable of precisely measuring the thermal noise power in the silence period illustrated in FIG. 1. With reference to FIG. 4, errors involved in measuring the thermal noise power of the silence period will be described.
FIG. 4 is a timing diagram illustrating power measured for the non-silence period and the silence period in the UPMB of the BS that will be referred to for describing power measurement errors.
In an actual 1xEV-DO system, there is no power artificially loaded on the reverse link for the silence period, as described before with reference to FIG. 1. The reason for setting the silence period is to precisely measure the thermal noise power and to appropriately control the data rate of the reverse link based on the measurement. When the average calculator 302 is implemented as an integrator however, it cannot follow rapid power changes, such as the square wave illustrated in FIG. 1. If a power signal is presented in the form of a square wave, the integrator will output power values that reflect a parabola as indicated by the bold line shown in FIG. 4. Therefore, a power measurement error is generated, as illustrated in FIG. 4. This error varies with the time constant of the average calculator 302. If the time constant is greater than 26.67 ms, 55.33 ms, or 80 ms, which are the duration's of a silence period as provided in the 1xEV-DO system, the thermal noise power cannot be measured accurately.
An inaccurate thermal noise power measurement has adverse effects on ROT assignments, leading to inappropriate control of the reverse link. As a result, there is no reason for setting the silence period. Consequently, communication quality is degraded for users in other sectors or BTSs, or users within the same sector or BTS.
Recently, techniques have been developed that measure the ROT accurately by separating the silence period and the non-silence period. However, many mobile communication systems have been deployed and are providing services which do not or cannot incorporate these techniques for ROT power measurement. Hence, substituting the UPMBs 300 in BTSs will cost a great deal and waste time as well.